Although a solar cell is a typical environment-friendly device, and the need for sustainable and environment-friendly alternative energy increases, there is a difficulty in commercialization of solar cell due to low efficiency and high costs.
To solve this problem, there are some methods in use, for example, a method which changes or suitably controls the material, thickness or structure of a solar cell, a method which forms an antireflective surface on the surface of a solar cell to increase the amount of light that passes through, a method which converts wavelengths of incident light from unavailable to available for a solar cell, etc.
Among them, the most typical technology for boosting solar cell efficiency is minimization of the reflection of light. And the technology includes the method that deposits materials having different refractive indices in multiple layers on the surface of a solar cell, however, here we have not only some limitations against effectiveness in that the refractive index is likely to be influenced by some conditions of the incidence (wavelength, angle of incidence, polarization, etc.), but also we have some problems about adhesion, stability, or thermal suitability attributable to multiple layers with various materials (Patent Literature 1).
The technology of reflection minimization includes another method that induces a change in effective refractive index by forming a fine grating pattern, called as moth eye structure, on the surface of a substrate to minimize the reflection of light. And this technology can attain very good antireflective characteristics (Patent Literature 2), but such moth eye patterns have optical characteristics greatly depending on the angle of protuberances. So when the nano structures are damaged by mechanical friction, the shape is deformed, and as a consequence, optical transmission and anti-reflective properties greatly reduce. Then, the moth eye nano structures might become a barrier on the application to optical components and optical radiation industries.
Also, having been developed another technology other than reflection minimization for boosting solar cell efficiency, which includes a wavelength conversion layer with an energy down-converting material in order to convert light incidence from too high, unavailable to moderate, available for a solar cell, however, this technology only increases the amount of light in a particular wavelength band, and due to a self-annihilation phenomenon or a surface plasmon resonance (SPR) phenomenon, a larger optical loss occurs, and due to the problem such as increased thickness of the solar cell, there is a limitation in boosting solar cell efficiency.
To overcome the limitation for boosting solar cell efficiency by the wavelength conversion layer, here comes alternative that the wavelength conversion layers are stacked in multiple layers. But as mentioned above, the solar cell efficiency reduces due to various problems occurred by stacking in multiple layers (Patent Literature 3).